This invention relates to the field of catalytic oxidation of hydrocarbons to produce oxygenated hydrocarbons, and more particularly to an improved process for the preparation of maleic anhydride and other oxygenated hydrocarbons at enhanced productivity.
Conventionally, maleic anhydride is manufactured by passing a gas comprising n-butane and oxygen through a fixed catalyst bed tubular plug flow reactor containing a catalyst that consists of mixed oxides of vanadium and phosphorus. The catalyst may contain minor amounts of promoters or activators such as iron, lithium, zinc, chromium, uranium, tungsten, various other metals, boron and/or silicon. Other nonaromatic hydrocarbon starting materials which contain at least four carbon atoms in a straight chain, for example, 1-butene, 2-butene, 1,3-butadiene and mixtures thereof, can also be used in the process. The hydrocarbon starting material reacts with the oxygen to produce maleic anhydride and various by-products, including carbon monoxide and carbon dioxide.
The oxidation of n-butane or other of the above-mentioned starting materials is highly exothermic. Conversion of n-butane to maleic anhydride releases about 350 kCal/mole. Conversion to carbon dioxide, which is equivalent to combustion of the n-butane, releases about 650 kCal/mole. Thus, a substantial amount of heat must be removed in the course of the reaction. Conventionally, a shell and tube heat exchanger is used as a reactor, with the catalyst packed in the tubes, through which the reactant gases are passed. A cooling fluid, typically molten salt, flows over the outsides of tubes. Because the length to diameter ratio of the tubes is high, the reaction system approaches plug flow. The cooling capacity is substantially uniform throughout the reactor, but the rate of reaction varies widely with the concentration of hydrocarbon reactant and temperature. Because the reactant gases are normally introduced into the catalyst bed at a relatively low temperature, the reaction rate is low in the region near the inlet, despite the fact that hydrocarbon concentration is at its maximum at this point. As the gas temperature increases due to initial reaction, heat is generated at a rate that increases as a function of distance in the direction of gas flow, requiring the gas temperature to rise as a function of distance in order that the cooling rate per unit of tube length balances the rate of heat generation per unit of tube length. The temperature continues to increase with distance along the length of the reactor tube until a point is reached at which depletion of the hydrocarbon causes the rate of heat generation to slow, allowing the remainder of the reactor to operate at a lower temperature differential. Thus a point of maximum temperature is reached, which is generally referred to as the "hot spot" of the reactor. When the reactor is under proper control, the hot spot occurs in an intermediate region of the catalyst bed, and from this point downstream to the gas exit, the temperature typically declines.
Problems occur in operation of the reactor if the hot spot temperature becomes too high; and especially serious problems can arise if it propagates or migrates to the exit of the reaction zone. Typically, the salt bath cooling fluid is maintained at a temperature of about 380.degree. to about 460.degree. C. If the gas temperature exceeds about 500.degree. C., or the difference between the gas temperature and the salt bath temperature is greater than about 80.degree. C., the catalyst degrades at an accelerated rate due to sintering or other effects, resulting in a progressive decline in the productivity of the plant and, in some instances, the selectivity of the catalyst. Moreover, because the reaction rate constant increases exponentially with temperature, the reaction can run away if the gas temperature substantially exceeds a temperature 80.degree. C. higher than the cooling fluid. Additionally, higher temperatures tend to favor the complete oxidation of the hydrocarbon to CO.sub.2 and water. This not only reduces the yield and productivity of desired product, but the higher heat of reaction released in conversion to CO.sub.2 compounds the problem by further increasing the temperature.
Excessive deactivation of catalyst due to thermal degradation can cause the hot spot to migrate to the exit end of the catalyst bed. In this case, it becomes necessary to lower the hydrocarbon concentration or space velocity. Otherwise, the exit gas may contain sufficient unreacted hydrocarbon to create risk of an uncontrolled reaction in downstream equipment. Reducing the hydrocarbon or space velocity results directly in a loss of productivity.
Accordingly, it has been a high priority in the art to design and operate maleic anhydride and other catalytic oxidation reactor systems to control both the magnitude and location of the temperature peak, the so-called hot spot of the reaction. Efforts have also been directed to developing systems in which the reactor temperature profile is as even as possible, thereby allowing operation at higher average temperature for higher productivity.
Catalyst packs having graded activity in the direction of gas flow have been proposed to meet various objectives. Such catalyst systems generally include a region of relatively low activity where the so-called hot spot of the reactor occurs in order to minimize the temperature peak at that hot spot. This stratagem serves several purposes. First, it helps to protect the system against the runaway reactions that can occur if the hot spot temperature peaks too high. If the catalyst activity is relatively low in such region, the resultant reaction rate moderation prevents the temperature from rising as high as it otherwise generally would. Moreover, at a given temperature, the reaction rate is relatively low, so that self-acceleration into dangerous conditions is inhibited. Controlling the temperature peak favors the preferred reactions in competition with the high activation energy side reactions that produce CO and CO.sub.2. It also minimizes the rate of degradation of the catalyst, which increases with temperature. Additionally, it allows operation at higher than conventional temperatures upstream and especially downstream of the hot spot, without risking runaway reaction, thereby providing a higher overall rate of heat dissipation, equating to a higher rate of production.
Palmer et al. U.S. Pat. No. 4,342,699 describes a process for the manufacture of maleic anhydride using a fixed catalyst bed that is graded so that reactivity increases over a least a portion of the effective reaction zone length from minimum activity nearest the feed end of the reaction zone to maximum activity nearest the exit end. Maleic anhydride is removed from the reactor effluent and unreacted n-butane is recycled to the feed end after a purge for removal of inerts. The composition of the feed gas is on the hydrocarbon rich side of the flammability envelope. The combination of n-butane rich feed and graded catalyst is said to result in improved productivity of maleic anhydride as compared to processes that do not use this combination of features. Palmer et al. prefer that the entire effective length of the reaction zone is graded from minimum activity at the feed end to maximum activity nearest the exit end, but also contemplate grading only a portion of the effective length in such manner, providing a zone of high or intermediate activity at the inlet end to provide a preheating zone. Palmer et al. describe grading of catalyst activity by dilution with inert particles having a size and shape at least roughly similar to the catalyst pellets. Several other methods for achieving the desired activity gradient are mentioned. One is to employ a supported catalyst in which the proportion of support decreases, and correspondingly the proportion of active catalyst increases, from the minimum to the maximum reactivity zones. Another is to partially impregnate a support with catalyst. A third is to use different catalysts, or varying blends of different catalysts, in the individual reactivity zones.
Mummey U.S. Pat. No. 4,855,459 describes an improved process for the catalytic oxidation of various C.sub.4 hydrocarbons to maleic anhydride under conditions sufficient to provide a single pass conversion of at least 70% of the hydrocarbon fed. The catalyst is diluted with inert solid material effective to stabilize the maleic anhydride yield such that the average yield decay is less than 0.30% of the established maleic anhydride yield per month over an extended period of sustained operations. Alternatively, a supported catalyst is used and the proportion of active catalyst on the support increases from the maximum to the minimum dilution stage. In the process of the '459 patent, suitable configurations for the diluted catalyst pack are not narrowly critical and vary depending upon overall catalyst pack length, production rate, composition of the active catalyst, reaction conditions and the like. Nonlimiting examples include (a) a configuration in which the diluted catalyst pack is graded in dilution such that dilution decreases over at least a portion of the catalyst pack length from maximum dilution nearest the feed inlet end to minimum dilution nearest the exit end and (b) a configuration in which a first portion of the catalyst pack, proceeding from the feed inlet end to the exit end, has minimum dilution and the remainder of the catalyst pack is graded from maximum dilution nearest the feed inlet end to minimum dilution nearest the exit end. In the latter case the initial minimum dilution zone is relatively short and can serve as a preheating zone for the gas feed stream. The catalyst pack is graded in dilution such that the minimum dilution occurs within that region of the reaction zone which extends over the initial 50% of the length of the catalyst pack in which the hottest point of the reaction zone is located.
Smith and Carberry, "On the Use of Partially Impregnated Catalysts for Yield Enhancement in Non-Isothermal Non-Adiabatic Fixed Bed Reactors," The Canadian Journal of Chemical Engineering, 53, pp. 347-349 (1975) discloses the use of catalyst pellets that are partially impregnated with active catalyst in the oxidation of naphthalene to phthalic anhydride. The paper reports the results of varying both the fraction of pellet radius occupied by deposited catalyst and the total amount of catalyst deposited. Partial impregnation was found to improve yields. By comparison with partially impregnated catalyst, tests with fully impregnated catalyst required relatively small catalyst particles, higher pressure drop and consequently higher inlet gas pressure. This reference mentions the use of different catalyst in zones of the bed in which different steps of a multi-step reaction predominantly occur, but does not disclose the grading of catalyst impregnation or pellet size along the length of the reactor.
Buchanan and Sundaresan, "Optimal Catalyst Distribution and Dilution in Nonisothermal Packed Bed Reactors," Chem. Eng. Comm., 1987, Vo. 52, pp. 33-51 presents conditions for optimal loading with a two-dimensional reactor model and applies them to catalyst dilution in a butane oxidation reactor. The reference refers to catalyst dilution with inerts as an important special case of non-uniform catalyst loading, and notes that under some circumstances a chemically non-uniform catalyst loading may be preferable to simple physical dilution with inerts. In discussing certain other references that describe chemically non-uniform catalyst, Buchanan et al. speculate that the motivation to use them may depend partly on the convenience of the various catalyst preparation procedures or on considerations of catalyst longevity. They note that, for the vanadium catalyst used for C.sub.4 oxidation, higher phosphorus content in the vicinity of the hot spot may help stabilize the catalyst against deactivation. They further observe that one opportunity for chemical variation arises when there is an inverse relationship between activity and selectivity. In oxidation of both butane and butene, as the phosphorus content of the catalyst increases, overall activity declines but selectivity to maleic anhydride increases. Buchanan et al. present data showing the effect of various catalyst dilution schemes on yield of desired product.
Kerr U.S. Pat. No. 3,474,041 describes the addition of an organophosphorus compound for reactivation of mixed vanadium and phosphorus oxide catalyst for the oxidation of butane to maleic anhydride. Various means for introducing the organophosphorus compound into the catalyst bed are described including introduction of the phosphorus compound into the butane and oxygen containing feed gas to the reactor. Best results are said to be obtained by adding the organophosphorus compound after discontinuing hydrocarbon flow and blowing the reactivated catalyst with air prior to the re-introduction of hydrocarbon. The reference notes that the phosphorus compound can serve as a stabilizer as well as a reactivator for the catalyst.
Click, et al., U.S. Pat. No. 4,515,899 describes steam regeneration of phosphorus treated vanadium/phosphorus/oxygen catalyst for maleic anhydride. The reference notes that treatment of the catalyst with phosphorus compound reduces activity but increases selectivity, the loss of activity being compensated for by an increase in temperature of the reaction. The reference reports that, in practice, it is found that phosphorus compounds concentrate near the feed end of the reactor, thus requiring that the amount of phosphorus addition be limited. Addition of steam after treatment with phosphorus compound re-distributes the phosphorus compound more evenly through the reaction zone.
Edwards U.S. Pat. No. 4,701,433 applies both water and a phosphorus compound in situ in amounts sufficient to partially deactivate a portion of the catalyst. Edwards teaches that the addition of the combination of phosphorus compound and water serves to deactivate the region in which the hot spot of the reaction occurs, thereby moving the hot spot downstream and apparently allowing for reactivation of the region in which the hot spot previously occurred. A similar disclosure is contained in Edwards U.S. Pat. No. 4,810,803. Both references disclose the use of alkyl phosphates and alkyl phosphites for such purpose.
Edwards U.S. Pat. No. 4,780,548 also describes a process for reactivation of a phosphorus/vanadium/oxide catalyst for the oxidation of n-butane to maleic anhydride.
Although the literature is replete with publications which discuss various aspects and purposes of catalyst activity modification in the fixed bed catalytic oxidation of hydrocarbons, a need has remained for catalyst systems which provide for the manufacture of maleic anhydride with maximum productivity.
Productivity of a catalytic oxidation system for the manufacture of maleic anhydride can be defined by the equation: ##EQU1## where GHSV is the gas hourly space velocity (hr.sup.-1). Computations indicate that the productivity can be converted to metric units by applying the factor 16.0, leading to the relationship: ##EQU2## Molar yield in turn is the product of conversion and selectivity. Conversion is a function of a number of operating variables including, but not limited to, temperature, space velocity and active catalyst density in the reactor tube. Because the reaction rate constant is highly temperature dependent, molar yield and productivity necessarily depend on the ability to operate the reactor at a relatively high average temperature without suffering runaway reaction, excess CO.sub.2 formation or catalyst degradation due to excess hot spot temperature.
Pressure drop through the reactor system is another variable which materially affects the productivity and performance of the reaction system. To achieve the same space velocity at high pressure drop not only consumes mechanical energy, but requires a higher hydrocarbon partial pressure at the reactor inlet. Because the hot spot temperature and tendency to runaway may be highly sensitive to the partial pressure of the hydrocarbon reactant, high pressure drop may require the initial hydrocarbon content of the gas to be curtailed to reduce the parametric sensitivity of the system, thus adversely affecting the productivity.